1. Field of the Invention
The present invention relates to a bonding tool and, more particularly, to a bonding tool for connecting inner leads and outer leads of a film carrier or the like to bonding pads of a semiconductor element and wirings of a printed wiring board, respectively, by means of ultrasonic compression bonding or ultrasonic thermocompression bonding.
2. Description of the Related Art
The bonding pads of a semiconductor element are ordinarily connected to a lead frame or a printed wiring board using bonding wires. However, in order to deal with fine patterning accompanying the increase in the number of pins used, the connection by means of a tape-automated-bonding (TAB) system is also being employed. In the TAB system, the gang bonding method is generally used in which the inner leads of the film carrier are connected in a batch by the thermocompression bonding to the corresponding bonding pads of the semiconductor element. However, the size of the semiconductor elements is being increased year after year along with the advances in the integrated circuit technoology, and accompanying that the number of the pads is also being increased. Because of this, the batch connection by the gang bonding method is becoming increasingly difficult.
For this reason, the single point bonding system in which the inner leads are connected one by one to the corresponding pads is being employed. The connection by such a system is carried out by an ultrasonic compression bonding system or an ultrasonic thermocompression bonding system.
FIG. 8 shows an ultrasonic bonding apparatus according to the prior art single point system. An ultrasonic vibrator 100 and a bonding tool 102 are united by a horn 101, and ultrasonic vibrations generated by the vibrator 100 are transmitted to the bonding tool 102 by the horn 101. In this example, the ultrasonic vibration takes place in the lateral direction in a plane parallel to the plane of the paper, and accordingly, the tool 102 is vibrated also in the lateral direction, as shown by an arrow 200. In bonding, a semiconductor element (chip) 111 placed on a stage 110 is moved so as to be positioned within the so-called device hole of a carrier tape 114, and then a bonding pad 112 of the chip 111 and an inner lead 113 are aligned. As a view, seen in the direction of arrow 115 in FIG. 8A, given in FIG. 8B shows, the inner lead 113 is pressed by the tool 102 against the pad 112 by lowering the horn 101, and the two pieces are connected by subjecting them to ultrasonic vibrations. It is to be noted that in FIG. 8B the tool 102 vibrates in the direction perpendicular to the plane of the paper. In addition, the tip of the tool 102 is machined to have a wedge shape, as shown in the figure, in order to prevent it from making contact with the adjacent inner leads 113.
Accompanying the improvement in the integrated circuit technology, the number of required pins, in other words, the number of bonding pads has been increased, as mentioned above, and the pads 112 are being provided on all of the four sides of the chip 111. In this case, for pads provided along the sides of the chip 111 not illustrated in FIG. 8A their connection with the inner leads 113 is achieved by vibrations in a plane parallel to the plane of the paper.
Each inner lead and the corresponding pad are connected in the manner described above. However, in order to obtain a satisfactory electrical and mechanical connections between them it is necessary to efficiently transmit the ultrasonic vibrations to the contact plane of the inner lead 113 and the pad 112. If the tool 102 makes contact with other inner leads, ultrasonic vibrations attenuate, and as a result, intended connection cannot be obtained.
In the case depicted in FIG. 8 the spacing between the pads (pad pitch) is larger than the width of the tool 102, so there will occur no problem of contact even if the semiconductor chip is of the so-called quad flat package (QFP) type. However, when the pad pitch becomes small due to the demand for larger number of pins, the tool 102 make contact with the adjacent inner leads, as is clear from FIG. 8C, in particular. Therefore, it is necessary to machine the tip of the bonding tool 102 in a thin shape. Moreover, in order to deal with the quad-flat-package (QFP) type semiconductor chips having a small pad pitch, the tip of the tool 102 has to be made thin not only when seen from the front side, as in FIG. 8B, but also when seen sideways as in FIG. 8C. Hence, in a bonding tool using ultrasonic vibrations for QFP type semiconductor chips with small pad pitch, it is conceivable to give its tip the so-called bottleneck shape when seen from the front (rear) direction as well as sideways.
In the actual ultrasonic bonding, however, ultrasonic vibrations are given while applying a load to the tip of the tool so as to press the inner lead 113 onto the pad 112. Because of this, if the tip of the tool 112 is given a bottleneck shape in a simple minded manner, the ultrasonic vibrations to be transmitted to the tip part will be attenuated significantly due to the load for pressing. Substantially, no vibration is transmitted to the tip part, and no connection can be achieved.